The all-new 991 Porsche 911 is already up and away here in Australia, but the range-topping Turbo variant isn't expected to debut until early next year.

Development on the iconic 911 Turbo is well into its final stages though; a pre-production prototype of the German supercar has been spotted during shakedown testing at the famed Nurburgring this week.

Although it's a clean-slate design, the 991-generation 911 Turbo will follow the template established by its predecessors and the new Carrera 4 - fatter fenders, intercooler-ducting in the rear quarter panels and a prominent rear spoiler.

There's also the usual heavily-vented rear bumper with integrated quad-exhaust outlets, and larger radiator openings in the front bumper.

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Powertrain details have yet to be announced, but an evolution of the current 997 Turbo's 3.8 litre flat six is likely to see service in the 991 Turbo.